Apparatus for increasing the yield and the production potential of urea reactors

ABSTRACT

Urea is formed by the synthesis of ammonia with carbon dioxide at high pressure and temperature in an internal space of reactors through which flow a liquid phase cocurrently with a gas phase. The reactors are divided into compartments to avoid excessive mixing of the entire liquid phase and to allow the intermittent redistribution of the gas in bubbles of a suitable size for increasing the exchange of heat and matter between the two phases. At each passage from one compartment to the next, the liquid phase and the gas phase are made to flow on separate routes and are distributed in each compartment with a continuous, permanent, and even flow.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns increasing the yield while at the same timeincreasing the production potential of reactors for producing urea,formed by synthesis between ammonia and carbon dioxide at high pressureand temperature. The reactors include an internal space for cocurrentfluid flow by a liquid phase and a gas phase, the internal space beingdivided into compartments to avoid excessive mixing of the liquid phaseand to allow for intermittent redistribution of gas in bubbles of a sizesuitable for increasing heat and matter exchange between the two phases.

The invention also concerns a device including seven perforated flatbaffles or plates distributed transversely in a cylindric reactor shellfor creating compartments therein, through which flow gas and liquidcontaining urea.

2. Description of the Related Art

In modern reactors for the synthesis of urea, two phase (a gas phase anda liquid phase) flow cocurrently inside the cylindrical shell of thereactor (which is under pressure). The shell is divided intocompartments by seven perforated plates. The purpose of thisconfiguration is to avoid excessive mixing of the entire liquid phasecontained in the reactor, which would tend to turn it into a completemixing reactor, thus reducing the urea yield. The amount of mixing isreduced by dividing the reactor into several stages by means of sevenplates. The behavior of the liquid phase approaches that of the behaviorof a piston flow reactor, which is notoriously the most favorable forkeeping the urea yield relatively high.

By dividing the reactor into seven stages by means of perforated plates,it is also possible to redistribute the gas which flows upwards alongthe column intermittently in smaller bubbles more suitable forincreasing heat and matter exchange between the two phases. In effect,the rising showers of bubbles are subjected to coalescence phenomenawhich progressively increase the size of the bubbles, thus reducing theexchange surface between phases. This negative phenomena is partlycompensated for by the redistribution brought about by the perforatedflat plates.

However, the cocurrent flow of gas and liquid over each perforated plateproduces some adverse effects on both the heat and matter exchange andthe urea yield. The latter is due to a diminution of the flow ofreagents into the liquid phase, as well as to the reduction intemperature because of the smaller exothermic reaction and also to thereduction in the liquid hold-up in the reactor. In effect, steam andliquid cannot go through the perforations in the flat bafflessimultaneously, but are forced to do so alternately by means of formingshowers of steam bubbles, separated by liquid pistons in continuousphase. Such an arrangement, as compared with a uniform distribution ofthe bubbles with the same amount of steam brings about a higherconcentration of bubbles inside the showers alternating with the liquidpistons. The result is a significant increase in coalescence of thebubbles between one plate and the next (increasing their average size),a reduction of the steam/liquid surface, and a worsening of the gasphase/liquid phase exchange since less steam is exchanged with theliquid phase. The volume available for this phase is also reduced (andthe temperature it has reached is also lowered). Moreover, between theplates and the cylinder there is in general a circular slit throughwhich part of the steam may go with less exchange efficiency. All thiscauses a reduction of the urea yield, compared with the yield obtainablewith a uniform distribution.

Also, the possibility of increasing the production potential in reactorsin existing plants is limited. Generally, the reactor, due to itspotential liquid phase capacity, is very large compared to the nominalurea production required of it, and this fact would lend itself, inprinciple, to possible increases in production with an almost constanturea yield. However, urea yield drastically worsens as the gas andliquid capacity increases, because of the poor distribution of gas, thesize of the bubbles due to parasitical coalescence, the matter and heatexchange between phases, the effective volume left to the liquid phase(in which the reaction forming the urea takes place). Accordingly, ureaproduction does not increase proportionately to the increase in totalliquid and gas capacity.

SUMMARY OF THE INVENTION

The main purpose of this invention is to provide a method to eliminatethe above-mentioned drawbacks and to increase the synthesis reactionyield and the potential of urea reactors.

Another purpose of the invention is to provide particularly simple andefficient devices to put into effect the method. In every transfer fromone compartment into the other the gas and liquid phases are made toflow in mutually separate and distributed ways, each with a continuous,permanent and even flow.

In a preferred embodiment, a device is provided including perforationsin plates having dimensional and/or shape gradients which create areafractions which permit the flow of liquid in the substantial absence ofgas bubbles. The bubbles sliding and being evenly distributed andconcentrated in an area which would be hard to reach by the liquid.

Therefore, according to an aspect of this invention, the reactor'sperforated plates are made in such a way as to allow the more evendistribution, with a permanent flow, of the gas bubbles avoiding theircoalescing between plates and the adverse effects of the two-phasemovement discussed above with showers of bubbles alternating with acontinuous liquid flow. The result is an increase in urea yield and ofthe reactor's production potential.

In a particularly simple and efficient and therefore preferredembodiment, the plates are differently perforated and shaped in such away as to allow the continuous and permanent flow of both the gas andthe liquid, both flowing along routes which are mutually separate anddistributed through each plate. The size of the perforations isdifferent for the area of the plate for the flow of liquid and the flowof gas. The size of the perforations in the areas intended for the flowof liquid being such as to impede the passage through them of gasbubbles together with the liquid, but rather to favor sliding of thebubbles toward zones intended for the gas flow. The area for the flow ofthe two phases are distributed in such a way as to ensure the evendistribution of the gas bubbles through said shaped perforated plates.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects and advantages of the invention will be made moreclear by the description of the embodiment represented in the drawingsin which:

FIG. 1 shows a schematic and partial longitudinal cross-sectional viewof a multi-compartment reactor in accordance with the invention;

FIG. 2 is an enlarged scale view of the elements forming the plates ofthe reactor and circled and labeled A in FIG. 1;

FIG. 3 is a top view of an upper part of the plates;

FIG. 3A shows the side of the plates;

FIG. 4 is a front view of a plate formed by rectangular elements;

FIG. 4A is an enlarged view of the portion of the plates circled andlabeled A' in FIG. 4;

FIGS. 5 and 5A are enlarged views of perforated portions of upper partsof the plates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, R shows the central cylindrical shell of the urea reactor andC1, C2, C3 are three transversal compartments created by the threeplates SP1, SP2 and SP3. These are formed by the lozenge-shaped elementsEL1 . . . . ELn which in FIG. 2 are trapeze-shaped and which in FIG. 4are preferably rectangular.

FIG. 2 shows that every ELi element is shaped like a Greek key, with awall shaped like an upside down trapeze formed by the side or lesserbase 2 at the top, by the greater base at the bottom 3, by the twoslanting sides 4 and 4', and by airspace 5. According to the main aspectof the invention, in the embodiment shown in FIG. 2, on the two slantingsides 4 and 4' there are perforations Fi larger than perforations fi onwall 2. Preferably, perforations fi have a diameter which is between 1and 3.5 mm, most preferably, about 2 to 3 mm, while the largeperforations Fi have a diameter which is almost twice that of fi, i.e.,2 to 7 mm, perforations Fi being preferably 3 to 6 mm.

FIG. 3 shows a top view of a wall 2 of a rectangular or trapeze-shapedelement EL'i.

FIG. 4 shows the front view of a flat baffle, perforated and formed byelements from EL'1 to EL'n which are rectangular, i.e., with walls 4 and4' parallel with the reactor's axis.

Perforations fi on wall 2 are generally circular as in FIG. 5. On theother hand, perforations Fi on walls 4 and 4' may be substantiallyellipse-shaped as shown in FIG. 5A as O. They are characterized by agreater axis AM and by a smaller axis MI.

EXAMPLE

Operations have been carried out by simulation on a reactor whose modelhas supplied, under nominal design conditions, for a production of about1800 t/d, a yield of 64% on the total flow (liquid plus gas). If theyield were to be (improperly) estimated on the sole liquid phase, itwould be about 65.5%. By increasing production up to about 2300 t/d, adiminution in yield was noticed. This confirms, indirectly, a) that thereactor is far too big, b) that an improvement in transport processesinside the reactor would not only increase the yield, compared tonominal conditions, but would also improve yield which would bemaintained throughout notable increases of its potential.

Operations were then continued by simulating with a rigorousmathematical model conditions for a daily production of 2300 t. in theunmodified reactor. A yield (simulated) was obtained of 61.3% which, ifbased (improperly) on the sole liquid phase would yield 63.5%. Theabsence of a number of data under these conditions does not allow forimmediate confirmation, but it would seem that the effective yieldpredicted by the model under increased production conditions isreasonably close to reality.

Simulation (through the model) of introducing into the reactor newplates in the 16 meters of the lower tangential line of the reactor wasthen carried out. The model modified in accordance with the inventionhas shown (for the potential of 2300 t/d already achieved) a yield of65.8% (+4.5% absolute) in respect of the total flow (equal to,improperly, 66.7% on the liquid phase, but with a drastic reduction insteam).

It has also been confirmed that such yield can be maintained for furtherincreases in potential up to 2700 t/d (if that were made possible by theother equipment in the plant). This is a further advantage of theinvention.

Consequently, in theory, under the conditions taken into consideration,an increase in yield (simulated) of 4.5% absolute could be obtained andfurther increases in production would be tolerable.

Some elements resulting from experiments are given below.

In the description which follows, linear dimensions for baffles and forthe passage of liquids are indicative. If necessary for constructionpurposes, they can be varied by about 50-10%.

This also applies to the number of perforations per meter both where theliquid phase and the gas phase passes.

On the other hand, the size of the perforations for the gas phase mustbe considered unchangeable while those for the liquid phase arevirtually so.

Finally, it is confirmed that the number of perforations per square forthe gas phase and the liquid phase should be read as referred to theareas for the gas phase and the liquid phase and not to the total area(gas plus liquid).

Numbering of the plates (suggested, or actually existing, for that partwhich has not been replaced) begins at the lower tangential line (T.L.)of the reactor (not shown on drawings).

In the same way, when the plates are being installed they should berotated alternately (in respect of the dome directrix) by 60°-90°,insofar as fastening points allow.

It has been found that, on the whole, the following instructions shouldbe followed when making the plates:

the thickness of the metal should not be more than 3.5-4 mm, to permitpunching of the perforations;

the plates should be oriented alternately, for example, fixing them atright angles or at 60°;

perforations for the installation should be made on that portion of thesheet metal reserved for the passage of liquid;

perforations per square meter for the surface intended for the passageof gas should be understood as referring to the entire surface of thesheet metal, even that which after being folded becomes lateral(vertical);

perforations for the gas should be made in equilateral triangular linkswith a 24 mm pitch;

after the plate has been shaped it is essential that the baffles areclosed at the ends with vertical walls welded to the terminal sectionscreating a seal to avoid gas escaping from the sides;

to make up for the thinness suitable supports or stiffeners can beapplied to the sheet metal after perforating and shaping;

the gas should be introduced into the lower part through a horizontaltube with multiple perforations set at right angles to the baffles ofthe lowest plate: if this were not possible, it would become necessaryto arrange two plates, with baffles at right angles between them, closeto one another (300-500 mm) which would obtain the same result (sincethe lower plate would act as a distributor for the upper plate).

The plates should be at a distance of about 2.4 m.

Perforations for the gas could, in principle, be varied from one plateto the other, with a perforated area decreasing from bottom towards thetop; however, with the plates arranged according to the invention, it ispossible to maintain even perforations equal to the maximum perforationsrequired for the lower plate. This simplifies construction andinstallation procedures. In the upper plates, the vapors (gas) aregradually decreasing, resulting in an increase of the level of theliquids inside the domes and a decrease of the head of gas (part of theperforations for the passage of gas will therefore be used by theliquid).

In a preferred embodiment:

perforations for the gas (in the present case) were 2000 holes/(squaremeter of relative area) with a diameter φ=3 mm; arranged as anequilateral triangle (preferred arrangement), this means a distancebetween perforations, for example, of 24 mm (and a fraction ofperforated area, on the area intended for gas, of 1.4%) (FIG. 5);

perforations for the liquid (in the present case), 600 holes/(squaremeter of relative area) with diameter φ=8 mm, arranged as an equilateraltriangle with a distance of 43 mm. Where oval perforations are used(FIG. 5A), their axes are 4 mm-6 mm, interspersed in the same way (witha fraction of perforated area of about 3% of the area for the liquid).

Obviously, the metallic strips bearing the perforations, after beingfolded and installed, should be bolted one to another, leaving an edgefor the alternating overlap from one dome to the next.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

We claim:
 1. A reactor for producing urea from ammonia and carbondioxide at high pressure and temperature, said reactor having;aninternal space for containing cocurrent upward flow of a continuousliquid phase and a dispersed gas phase, said internal space beingdivided into a plurality of compartments to avoid excessive mixing ofthe liquid phase and to intermittently redistribute the gas phase inbubbles of a size suitable for increasing heat and mass transfer betweenthe liquid phase and the gas phase means for introducing ammonia andcarbon dioxide into a lower part of said internal space, and means forwithdrawing urea containing product from an upper part of the internalspace; a plurality of baffles creating said compartments, each bafflecomprising a plate, a plurality of elements having one of a rectangularcross section and a trapezoidal cross section trapezoidalcross-sections, said elements extending upwardly from said plate andbeing open at the bottom, each said element having:two perforated sidewalls defining a first area containing apertures of predetermined sizefor flow of the liquid phase substantially free of gas bubbles, and aperforated top wall defining a second area containing apertures ofpredetermined size for flow of the gas phase substantially free of theliquid phase, all said perforations in the side wall being larger insize than the perforations in the top wall.
 2. The device of claim 1,wherein the device comprises seven baffles.
 3. The reactor of claim 1,wherein each baffle comprises a plurality of elements having rectangularcross-sections.
 4. The reactor of claim 3, wherein the perforations inthe side walls have a diameter of 2 to 6 mm. and the perforations in thetop wall have a diameter of 1.5 to 3.5 mm.
 5. The reactor of claim 4,wherein the perforations in the side walls have a diameter of about 4.5mm. and the perforations in the top wall have a diameter of about 2.5mm.
 6. The reactor of claim 3, wherein the perforations in the sidewalls are oval and the perforations in the top wall are round.
 7. Thereactor of claim 1, wherein each baffle comprises a plurality ofelements having trapezoidal cross-sections.
 8. The reactor of claim 7,wherein the perforations in the side walls have a diameter of 2 to 6 mm.and the perforations in the top wall have a diameter of 1.5 to 3.5 mm.9. The reactor of claim 8, wherein the perforations in the side wallshave a diameter of about 4.5 mm. and the perforations in the top wallhave a diameter of about 2.5 mm.
 10. The reactor of claim 7, wherein theperforations in the side walls are oval and the perforations in the topwall are round.